What are the letters in a XAML Geometry attribute called?

Question

I am defining a XAML DrawingBrush resource and I want to draw a custom shape. I found the following GeometryDrawing example:

<GeometryDrawing Geometry="M0,0.1 L0.1,0 1,0.9, 0.9,1z" Brush="Gray" />

What do the M, L, and z letters in the Geometry attribute mean? What are they called?

Answer 1

This is a StreamGeometry. It uses something called Path Markup Syntax. M is start point, L starts a line, Z is close.